Navigate to a new Screen and Back and Pass Data

 import "package:flutter/material.dart";

void main()
{
  runApp(const MyApp());
}
class MyApp extends StatelessWidget
{
  const MyApp({super.key});
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      title: "CounterApp",
      debugShowCheckedModeBanner: false,
      theme: ThemeData(
        primarySwatch: Colors.grey,
      ),
      home: CounterApp(),
    );
  }
}
class CounterApp extends StatefulWidget
{
  const CounterApp({super.key});
  @override
  State<CounterApp> createState()
  {
    return CounterAppState();
  }
}
class CounterAppState extends State<CounterApp>
{
  var counter = 0;

  void increment()
  {
    setState(() {
      counter++;
    },);
  }

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(
        title: const Text("Counter App"),
        centerTitle: true,
        backgroundColor: Colors.grey,
      ),
      body: Center(
        child: Column(
          mainAxisAlignment: MainAxisAlignment.center,
          children: [
            Text("$counter",
              style: const TextStyle(fontWeight: FontWeight.bold, fontSize: 50.0),),
            const SizedBox(height: 30.0),
            ElevatedButton(
              onPressed: () {
                Navigator.push(
                  context,
                  MaterialPageRoute(
                    builder: (context) {
                      return NextScreen(index: counter);
                    },
                  ),
                );
              },
              style: ElevatedButton.styleFrom(
                fixedSize: const Size(350.0,50.0),
                backgroundColor: Colors.blue,
              ),
              child: const Text("Go to next screen",
                style: TextStyle(fontSize: 30.0, color: Colors.white),),
            ),
          ],
        ),
      ),
      floatingActionButton: FloatingActionButton(
        onPressed: () {
          setState(() {
            increment();
          },);
        },
        backgroundColor: Colors.blue,
        child: const Icon(Icons.add, color: Colors.white),
      ),
    );
  }
}
class NextScreen extends StatelessWidget
{
  NextScreen({super.key, required this.index});
  int index;

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(
        title: const Text("This is Next Screen"),
        centerTitle: true,
        backgroundColor: Colors.grey,
      ),
      body: Center(
        child: Column(
          mainAxisAlignment: MainAxisAlignment.center,
          children: [
            Text("$index", style: const TextStyle(fontSize: 50.0, fontWeight: FontWeight.bold),),
            const SizedBox(height: 30.0),
            ElevatedButton(
              onPressed: () {
                Navigator.pop(context);
              },
              style: ElevatedButton.styleFrom(
                fixedSize: const Size(350.0, 50.0),
                backgroundColor: Colors.blue,
              ),
              child: const Text("Go back",
                style: TextStyle(fontWeight: FontWeight.bold, fontSize: 30.0, color: Colors.white),),
            ),
          ],
        ),
      ),
    );
  }
}